The present invention relates to cellulose formate fibers and to fibers of cellulose regenerated from this formate, and also to processes for obtaining such fibers when they are used starting from liquid-crystal solutions of cellulose formate, i.e. of solutions having a high concentration of polymer.
Liquid-crystal solutions of polymers, in the field of spinning, are used in known manner for obtaining what are called xe2x80x9ctechnicalxe2x80x9d fibers having very high properties of tenacity and of tensile modulus, combined with low elongations at break, as has been shown in particular by patents U.S. Pat. No. 3,767,756, which relates to aramid fibers, or U.S. Pat. No. 4,746,694, which relates to aromatic polyester fibers. The spinning of liquid-crystal solutions based on cellulose also makes it possible to obtain cellulose fibers having high mechanical properties, as described, for example, in International Patent Application PCT/CH85/00065, published under the number WO 85/05115.
This application WO 85/05115, or equivalent patents EP-B-179 822 and U.S. Pat. No. 4,839,113, disclose obtaining liquid-crystal solutions of cellulose formate by reacting cellulose with formic acid and phosphoric acid. These solutions may be spun using what is called the xe2x80x9cdry-jet-wet spinningxe2x80x9d technique to obtain cellulose formate fibers, as well as regenerated cellulose fibers after regeneration treatment of these formate fibers. Compared with conventional cellulose fibers such as rayon or viscose fibers, spun from non-liquid-crystal solutions, the fibers described in WO 85/05115 are characterized, owing to the liquid-crystal nature of the spinning solutions from which they have originated, by a far more ordered or oriented structure, and hence by a substantially greater strength and modulus: their tenacities may vary, for example, from 50-60 cN/tex to 80-100 cN/tex, or even more, depending on whether they are formate fibers or fibers of cellulose regenerated from this formate, their initial moduli possibly attaining 3000 to 3500 cN/tex; their values of elongation at break, in return, are low, of the order of 3% to 4%.
To obtain these high-strength, high-modulus fibers, the coagulation step is performed in acetone. Now, acetone is a relatively costly, volatile product, which furthermore involves a risk of explosion which requires special safety measures. Such disadvantages are not peculiar to acetone, but common, at least for some of them, to numerous organic liquids used in the spinning industry, in particular as coagulating agents.
It was therefore entirely desirable to find an alternative to the use of acetone by replacing this acetone with a coagulating agent which would be more advantageous from an industrial point of view and easier to use, even at the expense of a reduction in the mechanical properties described previously.
It turns out that simply replacing acetone with water in the spinning process according to the aforementioned application WO 85/05115 results in fibers having very poor mechanical properties and which are of no real industrial interest: firstly their tenacity is clearly insufficient to meet xe2x80x9ctechnicalxe2x80x9d applications (such as for reinforcing pneumatic tires), and secondly their elongation at break is too poor for xe2x80x9ctextilexe2x80x9d applications (for example in the clothing industry), which require high values of elongation at break.
Applicants have discovered during their research that it was possible to obtain, due to a novel, specific spinning process, a cellulose formate fiber meeting the requirements of textile application, i.e. having a high elongation at break of at least 8%. This specific spinning process, used starting with the liquid-crystal solutions based on cellulose formate described previously, uses-water as a coagulating agent, and not acetone: it is therefore particularly advantageous.
Thus, it is henceforth possible, starting from the same liquid-crystal solutions based on cellulose formate, and modifying the nature of the coagulating agent, to obtain either technical fibers of high mechanical strength by coagulation in acetone, or textile fibers of high elongation at break by coagulation in water. The existence of these two complementary methods specifies the field of industrial application of the base invention described in the aforementioned application WO 85/05115.
Consequently, a first object of the invention is a cellulose formate fiber of liquid-crystal origin, characterized by the following properties:
a) under an optical polarizing microscope, its filaments have a banded structure typical of its liquid-crystal origin;
b) it satisfies the following relationships:
DSxe2x89xa72; Arxe2x89xa78; Te less than 45; Mi greater than 500; Er greater than 10, DS being the degree of substitution of the cellulose with formate groups in the fiber (in %), Ar the elongation at break (in %) of the fiber, Te its tenacity (in cN/tex), Mi its initial modulus (in cN/tex) and Er its energy at break (in J/g).
Preferably, the elongation at break Ar is at least 10%, more preferably still at least 12%.
This combination of properties is quite unexpected for a cellulose formate fiber of liquid-crystal origin, because the highly ordered nature of the initial liquid-crystal solutions is a priori incompatible with a fiber, the elongation at break of which may achieve values of 8 to 12%, no document of the prior art furthermore describing a cellulose formate fiber of liquid-crystal origin having an elongation at break which exceeds values of 4 to 4.5%.
The invention also relates to a process for obtaining the above cellulose formate fiber, by spinning and coagulation in water of a liquid-crystal solution of cellulose formate using what is called the xe2x80x9cdry-jet-wet-spinningxe2x80x9d method, this process being characterized in that it comprises the following steps:
a) the starting point is a solution of cellulose formate in at least one phosphoric acid, this solution being optically anisotropic at ambient temperature and at rest;
b) this solution is extruded through a spinneret at a temperature (Tf) such that the solution is optically anisotropic if it is at rest;
c) the solution thus extruded is then stretched through a non-coagulating layer, preferably air;
d) then, by passing through coagulation means, the solution thus stretched is coagulated in water, the temperature (Tc) of which is greater than 5xc2x0 C., the tensile stress ("sgr"c) to which the fiber is subjected upon emerging from the coagulation means being kept at a value of less than 5 cN/tex;
e) the fiber thus formed is washed, and may possibly be dried.
Preferably, the concentration of cellulose formate in the spinning solution is at least 16%, more preferably equal to or greater than 22% (% by weight). Such concentrations, which are unusual for spinning cellulose fibers for textile purposes, such as viscose fiber, for example, arc particularly advantageous in that they make it possible to reduce the quantity of solvent in the spinning solution; furthermore, they make it possible to use the spinning process of the invention at particularly high speeds.
The cellulose formate fiber of the invention may be used as the precursor of a regenerated cellulose fiber which forms another object of the invention.
The invention relates in particular to a fiber of cellulose regenerated from cellulose formate, of liquid-crystal origin, having itself a high elongation at break: this fiber is characterized by the following properties:
a) under an optical polarizing microscope, its filaments have a banded structure typical of its liquid-crystal origin;
b) it satisfies the following relationships:
0 less than DS less than 2; ARxe2x89xa78; TE less than 45; MI greater than 500 greater than 10, DS being the degree of substitution of the cellulose with formate groups in the fiber (in %) AR the elongation at break (in %) of the fiber, TE its tenacity (in cN/tex), its initial modulus MI (in cN/tex) and ER its energy at break (in J/g).
The invention also relates to a process for obtaining the above regenerated cellulose fiber, comprising the following steps:
a) the starting point is a cellulose formate fiber according to the invention;
b) this formate fiber is treated by passing through regeneration means, washing means and then drying means, such that the tensile stresses ("sgr"r) to which the fiber is subjected at the entry to each of said means are less than 5 cN/tex.
The invention also relates to the use of the fibers of the invention in the field of textiles, in particular in clothing or furnishing, these fibers possibly being, for example, in the state of woven or non-woven, or knitted continuous fibers, or in the state of short fibers (staple fibers).
The invention, and its advantages, will be readily understood with reference to the description and the non-limitative embodiments which follow.